Under certain operating conditions aircraft are vulnerable to accumulation of ice on component surfaces. If left unchecked, such accumulations can eventually so laden the aircraft with additional weight and so alter the airfoil configuration of the wings as to cause undesirable flying conditions. A wide variety of systems have been proposed for removing ice from aircraft during flight or for preventing its accumulation on the leading edge surfaces of such aircraft. These systems can be categorized in three ways: thermal, chemical and mechanical.
The mechanical category of deicing systems covers a wide range of devices, all of which distort the airfoil surface in some manner so as to shed ice from the airfoil surface. A subcategory of mechanical deicing systems are high impact mechanical deicing systems which utilize high surface accelerations (normal to the surface) and strain to break and debond accumulated ice. Representative of such high impact systems are the electro-expulsive deicer, the eddy current deicer, the pneumatic impulse ice protection deicer, and the electro-impact deicing system. An example of electro-expulsive deicing systems can be found in three disclosures discussed hereinafter.
In U.S. Pat. No. 3,809,341 to Levin et al., flat buses are arranged opposite one another with one side of each bus being adjacent an inner or obverse surface of an ice collecting wall. An electric current is passed through each bus and the resulting interacting magnetic fields force the buses apart and deform the ice collecting walls. The disadvantage of this system is that each bus operates on the outer skin of the airfoil and a predetermined skin deflection is required to provide a set level of ice removal. Operating power needs often result in bus areas substantially smaller than skin areas, thereby necessitating large force requirements in order to generate the needed amount of skin deflection. Such high skin deflections are believed likely to cause fatigue in the skin.
U.S. Pat. No. 4,690,353 to Haslim et al. discloses a system wherein one or more overlapped flexible ribbon conductors are embedded in an elastomeric material affixed to the outer surface of an airfoil structure. The conductors are fed large current pulses and the resulting interacting magnetic fields produce an electro-expulsive force which distends the elastomeric member and separates the elastomeric member from a solid body such as ice thereon. The conductors in a single conductive layer as disclosed by Haslim et al. have a serpentine or zig-zag configuration.
Commonly owned U.S. Pat. No. 4,875,644 to Adams et al. discloses an electro-expulsive deicing system wherein a plurality of expulsive elements are placed in different layers on the airfoil surface, with each element being comprised of electrically conductive members interconnected such that electric current flowing in the conductive members flows in the same direction in adjacent electrically conductive members in a first sheet-like array and also flows in adjacent electrically conductive members of a second sheet-like array in a direction opposite to the first.
Commonly owned U.S. Pat. No. 4,706,911 and 4,836,474 disclose a high pressure pneumatic impulse deicer comprised of an outer skin having a substantially elevated modulus, and an pneumatic deflection means to deflect the skin in a short time span.
Metal weathering surfaces are commonly used as a durable, aerodynamic material in high impact mechanical deicing systems as described above. By common aircraft practice, the metal surface is formed to cover the airfoil shaped leading edge section to extend aft and be supported or attached to a spar or stringer for a wing or tail upper and lower surface or a former/bulk head center line to aft for an inlet. The extent or length of a given representative deicing element is substantially less than the metal surface that covers it. The deicer element impulse that causes the ice removing metal surface acceleration is restrained by the metal surface curvature and metal strain since it is fixed in place at its attachment points. Additional restriction to the deicing action comes from any bonding mechanism attaching the metal outer surface to the airfoil either directly or indirectly through other deicer elements. The metal surface strain is affected by its modulus of elasticity and thickness.
Both elements of the deicing action are inhibited by increased stiffness of a metal surface formed over smaller leading edge radius airfoil sections. The ice de-bonding acceleration impact action can be limited by any or all of these factors in combination on a metal surface: surface curvature, size of element relative to metal surface size, attachment mechanism of metal surface to the airfoil, metal modulus of elasticity and thickness and size of the airfoil leading edge radius.
Efforts to improve such mechanical deicing systems have led to continuing developments to improve their versatility, practicality and efficiency.